Sensitizing dyes are added to photographic emulsions in order to impart spectral sensitivity beyond the intrinsic absorption range of silver halide. Sensitizing dyes are compounds that absorb light at wavelengths ranging from the near UV (ca. 400 nm) to the infrared (ca. 850 nm). Light that is absorbed by said sensitizing dyes results in electron injection into the conduction band of the silver halide, and ultimately in the formation of a latent image. In this way, the spectral response of photographic emulsions is extended to the spectral region covered by the absorption of the dye.
When used to impart spectral sensitivity to a photographic silver halide grains, the sensitizing dyes are usually removed during the processing steps (developing, fixing, and washing). The current trend is towards decreasing these processing times, since this has the advantage of reducing the amount of effluents. However, removal of the sensitizing dyes is often less efficient with shorter processing times. Residual sensitizing dyes, which remain in the emulsions after processing, cause unwanted coloration (dye stain) of the photographic material. In addition, some sensitizing dyes have a high propensity towards aggregation and/or have poor water solubility and as a result would require excessive processing times for complete removal.